Breather Membrane basics

Nilvent breather membrane

In our product literature we often recommend using a breather membrane, such as nilvent, between the insulation and the tiles / slates on pitched roofs.

But what exactly is a breather membrane and why is it there?

The BBA defines them as follows:

a breathable membrane is a material which, in service conditions, is sufficiently permeable to water vapour transfer to adequately limit the risk of condensation in roofs.         

Basically, breather membranes are fitted to allow water vapour to escape to the outside of the building. The reason that we want the water vapour to escape is due to the difference in humidity between the outside and the inside of the building. This causes the moisture to move from one to the other. If this moisture hits a relatively cold and impermeable layer it may condense. This condensation can damage the fabric of the roof if allowed to collect.

In some constructions of pitched roofs, sarking felt can be such a layer which can cause condensation. Traditionally this is prevented by having ventilation in the roof, between the insulation and sarking felt layers, which allows the water vapour to escape before condensation forms, however studies have shown that ventilation directly above an insulation layer can reduce its thermal efficiency.

By fitting a breathable membrane this issue is avoided – the water vapour can pass through this layer without the need for ventilation. Research suggests that an unventilated roof is more energy efficient than a ventilated one, if it is properly sealed. To aid this we recommend fully sealing all joints in the breather membrane with tape. This will help prevent accidental air-leakage, making the roof as energy efficient as possible.

So basically a breather membrane:

  • allows water vapour to escape,
  • prevents condensation forming
  • allows for the creation of a more energy efficient roof.

As well as pitched roofs, breather membranes can also be used with timber frame walls – see our Kooltherm K12 brochure for more details. The principles are the same, water vapour escapes and condensation doesn’t form.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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